The adaptation 50 million years ago of a lineage of terrestrial mammals to aquatic environments is one of the most well understood events in mammalian evolutionary history. It’s also one of the most dramatic.
We often think of such adaptations in terms of gaining new genes and thus traits, allowing radical changes in lifestyle. New research, however, reveals that the adaptation to living in water was just as much about losing genes as it was about gaining them.
A team of researchers from the Max Planck Institute in Dresden, Germany, and the University of California in the US undertook a systematic survey of the genes lost as the organisms that would eventually become cetaceans (whales, dolphins and porpoises) split away from other mammal lineages.
This is because, write the authors, “gene loss not only can be a consequence of relaxed selection on a function that became obsolete but also can be a mechanism for adaptation”.
Where once a trait might have been crucial on land, in the water it may well serve only to hinder, or worse. Senior author Michael Hiller and colleagues note, for example, previous research that has indicated gene loss has helped streamline cetacean bodies by doing away with body hair.
On land, hair is often a necessary part of an organism’s heat regulation strategy, but in water it can cause drag, leading to inefficient swimming and all that comes with it, such as poor predator avoidance. In this case, the loss of genes producing keratin actually helped to adapt to the new environment.
In this study, the scientists discovered 85 genes that had been deactivated by mutations in both extant cetacean clades; odontocetes (toothed whales) and mysticetes (baleen whales).
This, they reason, indicates that these genes were lost before the two clades split and was thus a characteristic of their common ancestor, or ‘stem lineage’. It is this cetacean stem lineage that first took the plunge and transitioned to an aquatic lifestyle during the Eocene period.
Of the 85 genes identified, 62 have never been reported and the researchers have identified eight genes specifically that are likely to have been involved in the adaptation of the stem lineage to a fully aquatic lifestyle. These are implicated in a wide range of traits, from sleep to saliva.
Two of the lost genes, called F12 and KLKB1, were associated with blood coagulation. While these are vital on land, in the water they could lead to dangerous clotting inside the blood vessels, known as thrombosis.
F12, for example, causes clotting when it encounters foreign surfaces in the body. Losing this gene may have been beneficial for cetaceans because “nitrogen microbubbles, which readily form in the blood upon repeated breath-hold diving, may act as foreign F12-activating surfaces entailing harmful thrombus formation.”
Other lost genes help to reduce the chance of genetic mutation caused by DNA repair mechanisms working to rectify damage done by the high oxygen levels in the blood necessary for deep diving.
Loss of the genes MAP3K19 and SEC14L3 might help prevent scarring and the resulting loss of elasticity in cetacean lungs which, unlike humans and other terrestrial mammals, collapse during deep diving and explosively expand upon resurfacing. This elasticity helps cetaceans to renew 90% of the air in their lungs in a single breath.
The gene SLC4A9 is partly responsible for the production of saliva in terrestrial mammals, but your average dolphin or whale has little need of spittle which is why it has been lost in cetaceans.
Saliva helps to lubricate the mouth, break down starch and facilitate taste, all of which are less important in an aquatic environment. Who needs oral lubrication when your meal comes with a mouthful of seawater?
Beyond that, write the authors “the hyperosmotic marine environment necessitates strict housekeeping of freshwater resources in marine species; thus, freshwater loss via saliva secretion may be detrimental.”
Cetacean ancestors, as mammals who need air to breathe, also faced issues with regards to the mammalian sleep cycle. Just as humans tend to avoid napping face-down in puddles, cetaceans can’t just fall asleep in the ocean.
So the creatures of the cetacean stem lineage had to find a way to balance the need for sleep with the restriction of their new aquatic environment. As a result, they have a unique adaptation called ‘unihemispheric sleep’, which “allows one brain hemisphere to sleep while the awake hemisphere coordinates movement for surfacing.”
This adaptation was facilitated by the loss of several genes involved in the production and reception of the sleep hormone melatonin. This helped to “decouple sleep-wake patterns from daytime,” which, argue the researchers, “may have been a precondition to adopt unihemispheric sleep as the exclusive sleep pattern.”
Hiller and his colleagues’ work, published in the journal Science Advances, helps us to understand not only the evolutionary history of the aquatic mammals, but also shines a light on the mechanics of evolutionary adaptation itself.
It’s not all about the relaxation of selection pressures on unnecessary traits, nor the gaining of new physiological or behavioural traits specific to a new environment. It is also about getting rid of old, once-useful traits that organisms are now better off without.
Stephen Fleischfresser is a lecturer at the University of Melbourne's Trinity College and holds a PhD in the History and Philosophy of Science.
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